The statement by Indian Prime Minister Manmohan Singh that the peace process between India and Pakistan cannot progress unless Islamabad cracks down on terrorism represents a hardening of Delhi's position.

For the first time since Tuesday's serial bombings in Mumbai, the Indian government has pointed a finger at its long-time rival.

Until now, Indian officials had been cautious in their comments - broadly describing the attacks as the work of terrorists but refusing to go any further.

But on Saturday, Delhi said that "no date had been set" for a meeting between foreign secretaries of the two countries that was scheduled to take place next week.

Indian Foreign Secretary Shyam Saran said that India was still committed to the peace process, but it had been made "more difficult" by the bombings.

Two Indian MPs have also cancelled their visit to a Commonwealth parliamentarians meeting in Islamabad.

These developments may be relatively small scale, but they nevertheless provide the first indication that the peace process is in jeopardy.

Militant cells

Off the record, some senior intelligence officials were more forthcoming, saying that Islamic groups with links to Pakistan were involved.

One report in an Indian newspaper on Friday even said that National Security Advisor MK Narayanan had briefed the cabinet on Thursday, saying there was little doubt on that score.

But now Prime Minister Manmohan Singh has publicly suggested that the blasts were carried out by militant cells supported from within Pakistan.

No group has been as yet named as having carried out the coordinated attacks on Mumbai's commuter railway system last Tuesday.

Manmohan Singh is coming under increasing pressure

But many believe that only a few groups have the expertise to execute them.

The key suspect is the Lashkar-e-Toiba, a Kashmiri militant group based in Pakistan and banned by both countries.

The group has been blamed for similar attacks in the past - in particular an attack on India's parliament in 2001 that brought both countries to the brink of war.

Lashkar has already denied having anything to do with the Mumbai blasts but few in India will take that at face value.

Radical Islamic groups within India are also under investigation for any role they may have played, including the Students Islamic Movement of India (Simi), a hardline group based in north India which has also been outlawed.

Although Simi has been implicated in attacks in the past, no member of the group has ever been charged, let alone convicted.

Pakistan blamed

Ever since India and Pakistan embarked on a peace process in January 2004, there has been a noticeable improvement in the tenor of their exchanges.

In the past it was routine for Delhi to blame Pakistan, particularly its intelligence services, over violent attacks in India.

After the attack on India's parliament, the angry exchange of rhetoric so soured relations that the two countries came close to an all out war.

President Musharraf has limited influence over Islamic hardliners

But things have improved since then and India has been far less aggressive in its comments since.

Pakistan was also one of the first countries to strongly condemn the bombings.

So why did India's prime minister make his comments which, without doubt, represent a hardened stance?

With the investigation making little tangible progress, the Indian government is under increasing pressure to act and demonstrate that it is not a soft target.

Many people are angry that three days after the bombings, there appear to have been no significant arrests, nothing to indicate that the government was moving swiftly to crackdown on those responsible.

Other's believe that it is time India demonstrates it will not take things lying down, even if that means reading the riot act to its neighbour.

But there are serious doubts that even if groups in Pakistan were directly involved, that the country's leader General Pervez Musharraf can do much to confront them.

While some believe that at least a part of the Pakistani establishment is reluctant to act against militants fighting Indian rule in Kashmir, the reality is that the Pakistan government has little control over hardline Islamic groups largely based in the frontier tribal region along the Afghan border.